1. Field of the Invention
The present invention relates to a pixel circuit, more particularly to a pixel circuit for an organic light emitting diode (OLED) based display device.
2. Description of the Related Art
Organic light emitting diode (OLED) based display devices have the advantages of self-illumination, high brightness, fast response times, and wide viewing angles, and have been employed in various applications.
An OLED display device uses an array of pixel circuits capable of displaying different colors. Moreover, control of illumination intensities of the pixel circuits is performed sequentially through either rows or columns of the array. Each pixel circuit includes an OLED, and is operable for generating a driving current for driving the OLED thereof. Illumination intensity of light emitted by each OLED is related to a magnitude of the corresponding driving current.
Referring to FIGS. 1 and 2, a conventional pixel circuit includes an OLED 11, a first transistor 12, a second transistor 13, a third transistor 14, a fourth transistor 15, a fifth transistor 16, a sixth transistor 17, a first capacitor 18, and a second capacitor 19. Each of the transistors 12-17 is an n-type thin-film transistor (TFT).
The conventional pixel circuit receives a data signal, a first scanning signal, an enable signal, a complementary enable signal, a second scanning signal, a reference signal and a reset signal. Operation of the conventional pixel circuit may be divided into a compensation phase, an light-emission phase, and a reset phase.
In the compensation phase, a source of the second transistor 13 has a voltage of VDATA−VT, where “VDATA” is a voltage of the data signal and “VT” is a threshold voltage of the second transistor 13.
In the light-emission phase, a voltage “VOLED—A” at an anode of the OLED 11 and the threshold voltage “VT” of the second transistor 13 are coupled to a gate of the second transistor 13 through the second capacitance 19, such that a voltage “VG” at the gate of the second transistor 13 satisfies the relationships ofVG=VREF+(VOLED—A−VDATA+VT)f, andf=C2/(C2+CP)
where “VREF” represents a voltage of the reference signal, “C2” represents a capacitance value of the second capacitor 19, and “Cp” represents a capacitance value of a parasitic capacitor associated with the gate of the second transistor 13.
The second transistor 13 generates a driving current “IDRIVE” satisfying the relationship of
                              I          DRIVE                =                ⁢                              1            2                    ⁢          μ          ⁢                                          ⁢                      C            OX                    ⁢                                                    W                L                            ⁡                              [                                                      V                    REF                                    +                                                            (                                                                        V                          OLED_A                                                -                                                  V                          DATA                                                +                                                  V                          T                                                                    )                                        ⁢                    f                                    -                                      V                    OLED_A                                    -                                      V                    T                                                  ]                                      2                                                  =                ⁢                              k            ⁡                          [                                                V                  REF                                -                                                      V                    DATA                                    ⁢                  f                                +                                                      (                                                                  V                        OLED_A                                            +                                              V                        T                                                              )                                    ⁢                                      (                                          f                      -                      1                                        )                                                              ]                                2                    
where “W/L” represent a width-to-length ratio of the second transistor 13.
In an ideal scenario where the capacitance value C2 is significantly greater than the capacitance value Cp (i.e., C2>>Cp), “f” is substantially equal to one, and the aforesaid relationship may be simplified into IDRIVE≈k(VREF−VDATA)2, such that the driving current “IDRIVE” is substantially unrelated to the threshold voltage “VT” of the second transistor 13 and the voltage “VOLED—A” at the anode of the OLED 11.
In practice, however, it may be difficult to achieve the configuration of the aforementioned ideal scenario due to space constraints. Although the conventional pixel circuit is able to compensate, to a certain extent, influence of changes of the threshold voltage “VT” of the second transistor 13 upon the driving current “IDRIVE”, the driving current “IDRIVE” is still related to the threshold voltage “VT” and hence is still susceptible to influence of changes in the threshold voltage “VT”.